All fiber lines comprise some type of washing equipment to separate the liquor of the digestion from the pulp. Later on in the process a washer is provided to separate bleaching liquors after bleaching stages. There are a number of different types of washers operating according to different principles.
A well-known type of washer is the drum washer, where the pulp is dewatered on a rotating filter drum after the addition of washing liquid, which displaces the liquor remaining on the pulp web after the preceding process stage, for example a digestion stage or a bleaching stage. An underpressure inside the drum causes the displaced liquid to pass through a perforated metal sheet arranged on the rotating drum. A further development of the original drum washer is the pressurized displacement washer, where the filtrate, at an overpressure, is brought to pass through the metal sheet. The increase in pressure difference leads to a more efficient filtrate displacement.
According to a known design of a pressurized displacement washer, the drum is provided with compartments, extending in the axial direction of the drum and intended to be filled with pulp. The compartments are defined by walls in the form of bars arranged axially along the entire drum shaft, as well as a bottom formed by the perforated metal sheet. The compartmentalization of the drum ensures that the pulp cake does not break up and get transported away, but instead maintains the shape which is produced upon application of the pulp. The perforated metal sheet, on which the pulp is deposited, is located at a distance from the main surface of the drum, so that filtrate channels are formed in the space between the drum and the metal sheet. Along the circumference of the drum there are at least as many filtrate channels as pulp compartments.
In a drum washer, a plurality of different washing stages can be carried out, with separate addition of washing liquid to the different stages, and also re-cycling of filtrate from one stage for use as washing liquid in another stage. In order to achieve maximum washing efficiency, it is desirable that washing liquid intended for a particular washing stage is not transferred to a later washing stage. (Due to a pressure difference between the stages, the supplied washing liquid tends to be transported towards the lower pressure.) In order to be able to separate different washing stages, which are carried out in one or more washing zones of the drum, and forming stages, which are carried out in the forming zone of the drum, and discharge stages, which are carried out in a discharge zone of the drum (a zone for enhanced pulp concentration constitutes a first part of the discharge zone), the respective zones are sealed by longitudinal (i.e. axial) seals. These longitudinal seals are arranged between the rotary drum and the surrounding casing. The filtrates from the respective zones are separated by seals in a peripheral end valve arranged at one or both of the end walls of the drum.
A problem associated with drum washers of the type that has zones separated by means of longitudinal seals is that these seals are exposed to abrasion, wear and other stresses. The seals change over time, which affects the general wash performance in a negative manner and also leads to risks for leakage and production interruptions.
According to the prior art, there is a possibility for a working staff to make a manual adjustment of the longitudinal seals. The principle is to wheel the seal in the direction towards the drum until the staff perceives a sound which serves to indicate that the seal lies in close contact with the drum and thereafter back the seal an arbitrary distance. Such procedures are circumstantial, irregular and completely dependent on personal qualities of the working staff.
Accordingly, there is a need for an improved solution to the problem with seals that are worn and change over time.
One general object of the present invention is to provide an improved washing apparatus of the kind with a compartmented rotatable drum. In particular, the invention aims at accomplishing a more secure and more efficient seal mechanism of the washing apparatus.